The present invention relates generally to the field of electrochemical devices, and more particularly solid state electrochemical devices composed of one or more electrodes in contact with a solid state electrolyte and/or membrane.
Solid state electrochemical devices are often implemented as cells including two electrodes, the anode and the cathode, and a dense solid electrolyte/membrane which separates the electrodes. In many implementations, such as in fuel cells and oxygen and syn gas generators, the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as oxygen ions, sodium ions, or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material (xe2x80x9cMIECxe2x80x9d). In each case, the electrolyte/membrane must be gas-tight to the electrochemical reactants. In all of these devices a lower total internal resistance of the cell results in improved performance.
The preparation of solid state electrochemical cells is well known. For example, a typical solid oxide fuel cell is composed of a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (xe2x80x9ccermetxe2x80x9d), in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of an ionically/electronically-conductive metal oxide on the oxidant side of the cell. Electricity is generated through the electrochemical reaction between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves mass transfer and charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen). The contribution of these charge transfer steps, in particular the charge transfer occurring at the oxygen electrode, to the total internal resistance of a solid oxide fuel cell device can be significant.
Electrode structures including a porous layer of electrolyte particles on a dense electrolyte membrane with electrocatalyst material on and within the porous layer of electrolyte are known. As shown in FIG. 1, such electrodes are generally prepared by applying an electrocatalyst precursor-containing electrode material 102 (such as a metal oxide powder having high catalytic activity and high reactivity with the electrolyte) as a slurry to a porous (pre-fired; unsintered; also referred to as xe2x80x9cgreenxe2x80x9d) electrolyte structure 104, and then co-firing the electrode and electrolyte materials to densify the electrolyte and form a composite electrolyte/electrode/electrocatalyst 106.
Oxides containing transition metals such as Co, Fe, Mn, are known to be useful as oxygen electrodes in electrochemical devices such as fuel cells, sensors, and oxygen separation devices. However, if such compounds were to be used with typical zirconia-based electrolytes, such as YSZ, a deleterious reaction in the temperature range of 1000-1400xc2x0 C. typically needed to densify zirconia would be expected. The product of this reaction would be a resistive film 105 at the electrode/electrolyte interface, thereby increasing the cell""s internal resistance.
Similar problems may be encountered with sintering highly catalytic electrode materials on densified (fired) zirconia-base electrolytes since the sintering temperatures of about 1200xc2x0 C. to 1400xc2x0 C. are sufficient to cause the formation of a deleterious resistive film at the electrode/electrolyte interface.
In order to avoid deleterious chemical reactions, attempts have been made to use barrier layers, such as ceria, or to use chemically compatible electrolytes, such as ceria with such transition metal oxides. Also, it has been proposed to add an electrocatalytic precursor to a fired electrode/electrolyte composite, but only for a specific type of electrode material. Specifically, prior researchers have sought to fabricate electrodes with interpenetrating networks of ionically conductive and electronically conductive materials with subsequent infiltration of a catalytic electrode. However, this can hinder the performance of ionic devices, particularly at high current densities where ohmic drop due to current collection can lead to substantial efficiency losses.
It would be desirable to have improved techniques for fabricating high performance solid state electrochemical device oxygen electrodes composed of highly conductive materials.
The present invention addresses this need by providing an electrode fabricated from highly electronically conductive materials such as metals, metal alloys, or electronically conductive ceramics. In this way, the electronic conductivity of the electrode substrate is maximized. Onto this electrode in the green state, a green ionic (e.g., electrolyte) film is deposited and the assembly is co-fired at a temperature suitable to fully densify the film while the substrate retains porosity. Subsequently, a catalytic material is added to the electrode structure by infiltration of a metal salt and subsequent low temperature firing. The invention allows for an electrode with high electronic conductivity and sufficient catalytic activity to achieve high power density in an ionic (electrochemical) device such as fuel cells and electrolytic gas separation systems.
In one aspect, the present invention provides a method of preparing a layered composite electrode/electrolyte structure having a porous electrode in contact with a dense electrolyte membrane. The method involves contacting a mixture of particles of an electronically-conductive or a homogeneous mixed ionically electronically-conductive (MIEC) electrode material with a layer of an ionically-conductive electrolyte material to form an assembly having a layer of the mixture on at least one side of the layer of the electrolyte material. The assembly is sintered, and the sintered assembly is then infiltrated with a solution or dispersion of an electrocatalyst precursor.
In another aspect, the invention provides a solid state electrochemical composite electrode/electrolyte structure for a solid state electrochemical device having a porous electrode in contact With a dense electrolyte membrane. The electrode includes a porous structure of particles of an electronically-conductive or a homogeneous mixed ionically electronically-conductive (MIEC) electrode material, and an electrocatalyst dispersed within the pores of the porous structure.